Process for ferrochrome electroplating



United States Patent Q 2,988,492 PROCESS FOR FERROCHROME ELECTRO- PLATING Tadashi Yoshida, 15 Yarai-cho Shinjuku-ku,

Tokyo, Japan No Drawing. Filed Dec. 23, 1957, Ser. No. 704,281 Claims priority, application Japan Apr. 15, 1957 1 Claim. (Cl. 204-43) The present invention relates to a process for the electroplating of ferrochrome containing 5 to 40% chromium by use of an electrolytic bath, which contains an appropriate amount of ions of manganese or aluminum alone or in combination, and the principal constituent of which consists of chromic sulfate, ferrous sulfate, ammonium sulfate and urea, with the object of establishing anindustrial process for the electroplating of a high grade iron-chromium alloy.

The present inventor has already been granted a patent, entitled Process for the Electrodeposition of Iron-Chromium Alloys, the detail of said invention being as described in the specification of U.S. Patent No. 2,766,196, patented on October 9, 1956. The process of said US. Patent No. 2,766,196 is, however, liable to have disadvantages as follows: first the electroplated film tends to cause cracks or peeling due to a relatively high internal stress of the plated alloy, secondly black streaks are liable to occur on the surface of the plated film, thirdly favorable range of the bath composition is relatively narrow and so forth.

The present inventor has found, based on the results of many experiments, that the above-mentioned difficulties can be removed by addition of an appropriate amount of ions of manganese or aluminum alone or in combination to the electrolytic bath.

Citations more or less related to the present invention are as follows: Stoddard, Jr. (Transactions of the Electrochemical Society, vol. 84, p. 305, 1943), has shown that a favorable effect may be obtained by adding a small amount of manganous chloride to an iron electrolytic bath.

On the other hand, Miiller et al. (Zeitschrift fiir Elektrochemie, vol. 47, p. 135, 1941), have reported that a small amount of aluminum chloride, added to a simple hot ferrous chloride bath render deposits softer.

Such processes have evidently no direct relation to the electrolytic deposition of iron-chromium alloy.

Recently, Snavely et al. (US. Patent No. 2,693,444, granted 1954), have introduced a process for electrodeposition of chromium-iron alloy of relatively high chromium content, characterized by electrolysing at above 135 F. (57 C.) by use of an electrolytic bath containing magnesium sulfate and sodium sulfite, of which principal constituents are chromic sulfate and ferrous sulfate. Snavely et al.s bath must always contain magnesium sulfate and sodium sulfite. (in the contrary, an electrolytic bath used in the present invention never includes the radical sulfite as described later. Further, Snavely et al.s bath includes no urea at all, but the electrolytic bath according to this invention contains always urea as referred to later.

Further, comparing from the standpoint of an electrolytic condition, electrolysis is conducted with a bath kept at least at 135 F. (57 C.) according to the Snavely, et al.s process, while in the present invention the electrolysis is never performed at a temperature above 131 F. (55 C.) as described later.

Namely, a process according to Snavely et al. is very much evident to be entirely different from the present invention.

2,988,492 Patented June 13, 1961 "ice Details of the electrolytic process according to the pres ent invention is as follows:

Electrolytic bath In order to dissolve trivalent chromium, chrome-ammonium alum or green chromic sulfate solution is used. It is convenient to use either ferrous sulfate crystal or ferrous ammonium sulfate crystal in order to dissolve divalent iron.

The electrolytic bath contains essentially per litre from 37 to 200 gr. anhydrous chromic sulfate, from 29 to gr. anhydrous ferrous sulfate, from 32 to 330 gr, ammonium sulfate, from 60 to 264 gr. urea and from 0.05 to 88.74 gr. in total of at least one anhydrous sulfate of a metal chosen between divalent manganese and trivalent aluminum, whereby an aqueous solution is prepared, having a weight ratio 0.3 to 3.2 of trivalent chromium to divalent iron, and kept at least for 4 hours at a temperature from 25 to 55 C., after the preparation and then used in the electrolysis. The range of pH appropriate for the electrolysis ranges from 1.7 to 2.5, and sulfuric acid or alkali carbonate are used in order to adjust the pH-value.

Further, it is sometimes advantageous to use a relatively small amount of such additional agents in the bath, as an inorganic acid or its salt containing fluorine in the molecule, oxalic acid or its salt,'citric acid or its salt, hydrazine or its salt or an appropriate surface active agent alone or in combination.

Anode An appropriate iron-chromium alloy, separated from bulk of the bath with diaphragm prepared from acidproof cloth, is used. Y

Cathode The electroconductive object to be plated is used as the cathode.

Electrolysis The aforementioned electrolytic bath is maintained at a certain temperature from 25 to 55 C., and electric current is passed at a cathode current density of 7 to 40 a./dm. to effect electrolysis. It is sometimes preferable to perform electrolysis, under an appropriate filtering and stirring of the bath.

Further, it is preferable to use a reasonable amount of iron scrap hanging in the bath, in order to reduce the ferric ions produced, and also supply the bath with an appropriate amount of ferrous ion, by the spontaneous dissolution of iron scrap.

The present invention having been detailed in the foregoing, it is of course obvious that when the composition of the electrolytic bath or handling thereof conditions for electrolysis, etc. do not satisfy the above-stated conditions, no satisfactory results will be obtained.

Effects obtainable according to the present invention are as follows:

Efiects of manganese ions By addition of 0.02 to 11 gr. divalent manganese per litre of the electrolytic bath, primarily a compact, smooth plating having relatively low internal stress may easily be obtained, secondly an effect is obtained avoiding to some extent the disadvantage of having black streaks formed on the surface of plate, and thirdly a particularly high corrosion resistant alloy plating may be obtained.

In this case, generally 0.005 to 0.02% manganese is codeposited together with iron-chromium alloy.

Effects of aluminum ions By addition of 0.02 to 5.4 gr. trivalent aluminum per a sence By using an electrolytic bath with those ions added therein as in the above-stated manner, an excellent plating is obtained, having a relatively low internal stress, compactness, smoothness and less tendency to cause cracks and peeling. This means that a relatively thick and excellent quality plated film can easily be deposited electrolytically. Further, additional realized, as follows:

The range of composition of the electrolytic bath within which fine quality alloy plating can be obtained is advantages are remarkably enlarged by including in the bath two sorts of ions mentioned above, and even by including only one sort of ions. Furthermore, an effect is obtained, rendering the ageing condition of the bath to be relatively mild.

of ions are as follows:

Firstly, with respect to the bath composition, according to my US. Patent No. 2,766,196, the electrolytic bath may include per litre at least 100 gr. urea, and simultaneously it is remarkably difficult to provide a satisfactory quality and good adhesiveness, unless the weight ratio of trivalent chromium to divalent iron is above 1.3. In contrast to the above, optimum contents of urea in the present bath ranges from 60 to 264 gr. per litre. ,Moreover, when the weight ratio of the trivalent chromium to divalent iron contents is within a. range from 0.3 to 3.2, a good quality plating is always obtainable having a high adhesive power according to this invention.

Secondly, with respect to the ageing of the electrolytic bath according to my aforementioned US. Patent No. 2,766,196, a relatively strict control of ageing is necessary for the bath, until the ions therein including trivalent chromium reach an equilibrium at the electrolytic temperature, while according to the present invention, the electrolytic bath can attain a satisfactory result by holding it at least for 4 hours at a temperature relatively near the electrolytic temperature within a range from to 55 -C., with no strict control being required.

Effects of urea or matters with particular reference: The above relationship is as shown in the specifications for US. Patent No. 2,704,273 and US. Patent No. 2,766,196.

Examples of practice according to the present invention: Referring to the chemicals used for the preparation of the electrolytic bath, which will be further mentioned in the examples of practice, it should be understood that Cr (SO aq. (392 gr./l.) means a green aqueous solution containing 392 gr./l. of Cr (SO but no sulfite radical, prepared by'reducing .chromic acid solution with sulfur dioxide gas; ferrous Effects common to each of the respective two sorts 4 tcr,(so, 156.9, Peso, 60.8, N11, ,s0, 132.2, (NH2)2CO 9o, Mnso 7.6 gr./l.]

Cr (III)/Fe (II) 1.86, pH 2.1.

An aqueous solution is first prepared containing per litre the "above-mentioned constituents, which is then maintained at about 50 C. for 6 hours, and used for the electrolysis.

Anodes Commercial 18% ferrochrome alloy sheets, separated from bulk of the bath with diaphragm prepared from acid-proof cloth, are used.

Cathodes Brass cases of a large size lighter are used.

Electrolysis The temperature of the bath is :2" C. Cathode current density, 25 a./dm. v The electrolysis is carried out under air stirring with above-said condition, while an appropriate amount of iron scrap is hung in the bath.

Results By electrolysis for 9minutes, a good semi-bright ferrochrome plating of about 0.01 mm. thickness containing about 22% chromium, can be obtained.

' EXAMPLE 2 Electrolytic bath Cr (SO aq. (392 gr./l.) cc 300 Ferrous sulfate gr 139 Ammonium sulfate "gr-.. 132.2 Urea gr 150 'Aluminum sulfate gr 20 [Cr(SO 117.7, FeSO, 76, (NH SO 132.2,

(NH CO 150, A1 (SO 10.3 gr./1.]

Cr (III)/Fe (II) 1.12, pH 1.87.

An aqueous solution is first prepared containing per litre the above-mentioned constituents, which is then kept at about 40 C. for 48 hours, and used for the electrolysis.

. Anodes Commercial 18% ferrochrome alloy poles, separated from bulk of the bath with diaphragm prepared from acid-proof cloth, are used.

cathodes Copper plated steel parts of bicycle are used.

Electrolysis The temperature of the bath is 37:2" C.

Cathode current density, 15a./dm.

The electrolysis is carried out with above-said condition, while an appropriate amount of iron scrap is hung in the bath.

Results By electrolysis for 15 minutes, a good semi-bright ferrochrome plating of about 0.015 mm. thickness containing about 19% chromium, can be obtained.

EXAMPLE 3 Electrolysis is carried out similarly with aforementioned Example 2, excepting that the electrolytic bath used contains per litre thereof further compound as follows:

' Results A good result can be obtained, like Example 2.

I claim:

A process for electroplating a ferrochrome alloy containing from 5 to 40% chromium, comprising: preparing an electrolytic bath comprising an aqueous solution containing per litre substantially from 37 to 200 grams of anhydrous chromic sulfate, from 29 to 185 grams of anhydrous ferrous sulfate, from 32 to 330 grams of ammonium sulfate, from 60 to 264 grams of urea, and from 0.05 to 88.74 grams in total of at least one anhydrous sulfate of a metal selected from the group consisting of divalent manganese and trivalent aluminum, the ratio of the trivalent chromium to divalent iron ranging'between 0.3 and 3.2 to l; keeping the aqueous solution at References Cited in the file of this patent UNITED STATES PATENTS Snavely et a1. Nov. 2, 1954 Yoshida Oct. 9, 1956 OTHER REFERENCES Stoddard: Transactions Electrochemical Society, vol. 84 (1943), pp. 305-312. 

